[7.09] Organic Solid Matter as a Coloring Agent in Outer Solar System Bodies

Small bodies in the outer Solar System OSS, exhibit a range
of color, or slope of the reflectance in the photovisual
spectral region, ranging from neutral to very red, sometimes
with and sometimes without distinct absorption bands. These
objects range in geometric albedo from 0.03 to 1.0, with the
higher albedo objects typically showing clear evidence of
water ice. Water ice has also been found in a few objects
with albedo 0.1 or less. We explore here the identification
of the material or materials that color these icy and
non-icy surfaces through scattering models that incorporate
minerals, meteoritic material, and organic solids (tholins)
produced in the laboratory by energy deposition in ices and
gases. These models must match not only the color in the
photovisual region, but the spectral reflectance properties
throughout the near-infrared. Among some classes of objects,
such as Kuiper Belt objects, the coloring agent may be a
single material that is present in greater or lesser
abundance, thus accounting for the range in color from
neutral to very red. This may also apply to the Centaur
objects, the jovian Trojans, and the outer-main belt
asteroids, each taken as a separate class. If so, each class
may be colored to varying degrees by a different material,
or they all might be colored by a common material that is
widespread throughout the OSS, from 3 to 50 AU, and beyond.
In this paper, we model the reflectances of Kuiper Belt
objects, Centaurs, Trojans, outer MB asteroids, and
planetary satellites. Our models show that the reddest
surfaces cannot be colored by minerals or meteoritic
materials, but can be matched throughout the photovisual and
near-infrared by organic solids, specifically certain
tholins.

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